When Chief Executive Leung Chun-ying delivers his maiden policy address a week from today, there is little doubt that the fingerprints of the man tasked with leading the government's think tank will be all over it.

The Central Policy Unit is considered one of the most opaque bodies in government, and its role has continued to evolve in the 24 years since it was created by the colonial government. Drafting the chief executive's annual agenda-setting speech has always been a part of its work.

Few doubt that Shiu Sin-por - who put the CPU in the spotlight shortly after taking office when he said he saw part of his role as winning the battle for public opinion on behalf of the government - will be one of the chief writers of Leung's speech.

When former governor David Wilson gave the speech 24 years ago, that job fell to Leo Goodstadt, the founding director of the CPU.

And while the unit has faced a flurry of accusations that it is trying to extend its role beyond being the government's principal think tank, Goodstadt says he sees little change in its fundamental purpose, and that it still serves three masters - the chief executive (or governor previously), financial secretary and chief secretary.

"It is not the unit trying to do something [new] - that is not the case," said the 73-year-old Irishman, an academic, journalist and economist.

Shiu's efforts to expand the influence of the CPU have three dimensions, according to critics.

On the personnel level, human resources expect Sophia Kao Ching-chi - a supporter of Leung during his chief executive election campaign - was appointed as a full-time member of the unit to advise the government on its appointments to advisory bodies.

On the academic front, the unit regained control of an annual HK$20 million public policy research fund, previously overseen by academics. A significant funding source for social scientists who study local policies, from columbariums to cross-border low carbon transport, the scheme will now be overhauled to do "more timely and issue-specific public policy research", the unit said last month.

Thirdly, and most controversially, Shiu revealed plans to turn the unit into a machine for drumming up support for the government and monitoring public opinion.

Shui has defended the government for running print and television advertisements promoting controversial policies, such as the plan for new towns in the northeastern New Territories and an old-age living allowance. Neither policy had been endorsed by the legislature when the ads hit the streets.

In contrast to Leung's smooth-talking style, Shiu does not mince his words.

Asked whether the unit was a political instrument, he said: "We must be a tool to serve the government. We are not a chit-chat unit or a playgroup."

Such remarks attracted criticism from the likes of Anson Chan Fang On-sang, the former chief secretary turned pan-democratic politician. She said Shiu had forgotten that he was a civil servant, for whom the rule of thumb ought to be neutrality. Shiu's predecessor, Professor Lau Siu-kai, who headed the think tank for a decade, said the unit had neither the duty nor the ability to drum up public support for government initiatives.

But while the CPU's website lists the unit's responsibilities as conducting policy research, analysing and assessing community concerns and public opinions, and drafting the policy address, Goodstadt says that even in its early days the unit was about serving the government.

"The CPU was a fashionable thing in the 1970s all over the world. The UK government had one, and found it very useful, so in 1989 it was proposed for Hong Kong," Goodstadt recalled. "They did not want theoretical analysis … All they needed was to have some firemen."

One of the main tasks of the unit in those days, according to Goodstadt, was dealing with problems which the rest of the government "either didn't want to deal with, or didn't feel they had the expertise, or alternatively, didn't have the resources".

"The unit would refuse to take up a project - even if we were asked to - if we believed somebody else in the government could do it better," he said.

Another fundamental role was to alert its clients, the government's top three leaders, when a problem was brewing, or when a widely expected problem was likely to be minor.

One famous example of those "non-problems" emerged after the bloody crackdown on protesters in Tiananmen Square in 1989, when people believed investors would pull out of Hong Kong and the mainland, particularly Guangdong province.

"There was nobody in the government at the time who could match the connections that Vincent Cheng Hoi-chuen, Linus Cheung Wing-lam, and I had with the business community," Goodstadt said, referring to the leading figures at HSBC and Cathay Pacific in the late 1980s, respectively, who joined the unit on a full-time basis by secondment.

"A report was immediately given to our clients. [And we said] never mind the newspapers, never mind the estimates. Hong Kong business is not retreating in significant numbers from the mainland."

Cheng and Cheung were the first batch of elite businessmen to join the unit. Barry Cheung Chun-yuen, now an Executive Council member, followed in the 1990s, on secondment from McKinsey & Company.

Unionists Lee Cheuk-yan, from the pan-democratic camp, and Chan Yuen-han, a Beijing loyalist, were recruited as part-time members.

One thing the unit worried less about at the time was the issue of short-term public support.

"What is public support? They [the public] support excellent policy, outstanding programmes that solve their problems," said Goodstadt. "Criticism makes no difference. In the 1990s, not a single newspaper supported the British administration every day."

When Goodstadt left as planned at the handover in 1997, the unit was taken over by career civil servant Gordon Siu Kwing-chue, who stressed the importance of policy formulation. He left the post after 15 months and was succeeded by Edgar Cheng Wai-kin, a son-in-law of the late shipping magnate Pao Yue-kong.

In 2002, Professor Lau, a political scientist at Chinese University, took over, remaining with the unit for 10 years.

During Lau's reign, the unit was a low-profile research body devoted to analysing public opinions with polls. Its influence on the chief executive and other senior officials faded after the 500,000-strong July 1 rally against national security legislation in 2003.

Shiu, who headed the One Country Two Systems Research Institute think tank for two decades, is a close ally of Leung. He has a reputation as one of the most steadfast supporters of Beijing's Hong Kong policies. He takes over running the body which had a budget of HK$86 million last year, overseeing four full-time staff, 40 part-time staff and scores of researchers.

His stance prompted pan-democrats to dub him "king of the leftists".

Dr Li Pang-kwong, director of the public governance programme at Lingnan University, a part-time member of the unit in the mid-2000s, said the influence of the Central Policy Unit was largely related to its relationship with the chief executive.

"Goodstadt was trusted by Lord Wilson, and Shiu is a very close ally of Leung Chun-ying," Li said.

"That explains how the unit could be influential."

Li said many people still failed to understand the purpose of the CPU.

"The unit does conduct policy research, but on a small scale, making it incomparable to leading think tanks in Taiwan, Singapore, and the mainland," he said. "The CPU plays a big role helping its three customers, despite its members not being political appointments."

On the face of it there is a huge contrast between the work and methods of Goodstadt and Shiu, but Li said they were actually doing similar work with the same goal - getting the public on the side of the government.

"Shiu might have failed in explaining his initiatives. It is his presentation, compounded by his leftist background, that adds to the public suspicions of his work. But I do not see a huge difference in Goodstadt's and Shiu's understandings of the system."

This article appeared in the South China Morning Post print edition as CY's advisers in the spotlight